In an electrophotographic image forming method, copied sheets are generally obtained via fixation with heat, pressure or solvent vapor, after an electrostatic latent image is formed by irradiating a photoconductive layer with an optical image depending on documents, the electrostatic latent image is developed by attaching colored powder called toner having a polarity opposite that of the latent image onto the electrostatic latent image, and a toner image is transferred to a transferred material such as paper, if desired.
As for a process of developing an electrostatic latent image, toner particles charged with a polarity opposite that of the latent image are attracted via electrostatic attraction, and adhered onto the electrostatic latent image, (in the case of a reversal development process, toner having the same polarity as that of the latent image charge is used), but generally, examples of the method of developing the electrostatic latent image with toners include two main types of methods such as a method of employing a so-called two-component developer in which a small amount of toners is dispersed in a medium called a carrier, and a method of employing a single-component developer in which toner is singly utilized without using a carrier.
Further, the carrier constituting a two-component developer is roughly classified into a conductive carrier and an insulating carrier. Oxidized or non-oxidized iron powder is commonly utilized as the conductive carrier, but there is a problem such that in the case of a developer containing this iron powder as a component, a frictional charging property is unstable to toner, and fog is generated in a visible image formed by the developer. That is, when such the developer is used, a bias current is lowered by increasing electrical resistance of carrier particles, and a frictional charging property becomes unstable since toner particles are adhered onto iron powder carrier particles with long-term use. As a result, image density of a formed visible image is lowered, and fog is increased.
On the other hand, a carrier, in which an insulating resin is evenly coated on the surface of a carrier core material made of iron, nickel, ferrite or such, is commonly known as a insulating carrier. In the case of a developer in which this carrier is employed, there is an advantage of being especially suitable for a high-speed electronic copying machine in view of excellent durability, and a long duration of use and life since fusion of toner particles on the carrier surface is slight in comparison to that of a conductive carrier.
Resistance of a carrier is also possible to be adjusted by coating the carrier surface, since generally, resistance of a core material used for a coated carrier is low, and resistance of a material used for a coated layer is high. Thus, Disclosed is a method of preventing adhesion of the carrier to an edge portion in a developing region during high-bias development, as well as preventing adhesion of the carrier to an image region during low-bias development, via resistance adjustment of the carrier by a process in which resistance of the coated layer is adjusted by dispersing carbon black (Patent Documents 1 and 2, for example), or metal oxide such as tin oxide, titanium oxide or zinc oxide (Patent Document 3, for example) in a coated layer in order to adjust resistance with thickness of the coated layer. However, there remains a problem such that a carrier resistance gradually declines, and the carrier is adhered to an image region, since the coated layer is diminished in quantity via friction, loss or such even though initial carrier resistance is adjusted by such the method.
Also disclosed is a method in which the amount of carbon black is designed to be larger toward the surface of a layer, and the amount of a charge control component (the amount of organic tin) is also designed to be larger toward the surface of a layer in order to stabilize the carrier resistance and the charging ability (Patent Document 4, for example). The carrier resistance is theoretically stabilized, and the charging ability becomes stable when the carrier resistance and the charging ability are adjusted by such the method, but the charging ability is degraded when resistance of the coated layer becomes large. That is, when the charging ability is lowered toward the lower portion of a layer, there is a problem produced such that from a practical standpoint, contamination and layer wear of the carrier surface are generated by toner and external additives, whereby the charging ability is degraded.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 56-126843
(Patent Document 2) Japanese Patent O.P.I. Publication No. 62-45984
(Patent Document 3) Japanese Patent O.P.I. Publication No. 64-35561
(Patent Document 4) Japanese Patent O.P.I. Publication No. 7-160059